> 'Long flu' is not a thing (it's actually possible but it's rare, mild and short)

That is not at all demonstrated scientifically.

"Long Covid" took quite a while to be demonstrated and we were monitoring everything like a hawk. "Long flu" has no such monitoring.

As soon as we had the tools and started looking for correlations, the collateral damage that viruses do pops up (HPV and cancer; Epstein-Barr and MS; Herpes and dementia).

Given that, the Bayesian prior favors "Long Flu exists and we just haven't paid attention" rather than "Long Flu doesn't exist."

My suspicion is that we're going to find more and more of these correlations as our medical ability to deal with viruses improves.